Method for the denitration of exhaust gas

ABSTRACT

A method for the denitration of exhaust gas from a gas turbine or an internal combustion engine which comprises (a) reducing a part of the NO 2  constituting NO x  contained in the exhaust gas to NO by means of a reduction catalyst bed for reducing NO 2  to NO, the reduction catalyst bed being installed in flow communication with the exhaust gas outlet of the gas turbine or internal combustion engine, and (b) injecting NH 3  into the exhaust gas within an exhaust heat recovery unit and then decomposing and removing NO x  contained in the exhaust gas and now composed chiefly of NO, by means a denitration catalyst bed installed within the exhaust heat recovery unit to effect the catalytic reduction of NO x  with NH 3 .

FIELD OF THE INVENTION AND RELATED ART STATEMENT

This invention relates to a method for the denitration of exhaust gas from a gas turbine or an internal combustion engine.

Conventionally, NO_(x) (a general term for nitrogen oxides) has been removed solely by means of a dry denitrator using NH₃ as a reducing agent, without providing any NO_(x) reducing means at the outlet of a gas turbine or internal combustion engine. A denitration system installed in a conventional combined plant is explained below with reference to FIG. 4. An exhaust gas duct 2 from a gas turbine or internal combustion engine 1 is connected with an exhaust gas boiler 3 which is followed by an exhaust gas duct 4 and a smokestack 5.

In the conventional denitration system, a plurality of heat exchangers 8, 9 and 10 are disposed within the exhaust gas boiler 3, and a denitrator 7 packed with a NO_(x) reduction catalyst is installed between heat exchangers 9 and 10. Moreover, an NH₃ injection nozzle 6 for injecting a NO_(x) reducing agent such as NH₃ is disposed upstream of the denitrator 7 in the exhaust gas flow. NO_(x) reacts with NH₃ under the action of the NO_(x) reducing agent to decompose the NO_(x) to N₂ and H₂ O.

For gas turbines and internal combustion engines such as diesel engines, the proportion of NO₂ in NO_(x) tends to be higher than that of NO because the O₂ concentration in exhaust gas is high as compared with that in boiler exhaust gas. It has been found that a dry denitrator using NH₃ as a reducing agent, which has conventionally been employed for the removal of NO_(x), removes NO_(x) according to the following equations (1), (2) and (3):

    4NO+4NH.sub.3 +O.sub.2 →4N.sub.2 +6H.sub.2 O        (1)

    NO+NO.sub.2 +2NH.sub.3 →2N.sub.2 +3H.sub.2 O        (2)

    6NO.sub.2 +8NH.sub.3 →7N.sub.2 +12H.sub.2 O         (3)

While the reaction rates of equations (1) and (2) are almost equal, the reaction rate of equation (3) is much lower than those of equations (1) and (2). When the proportion of NO₂ in exhaust gas is high, NO and a part of the NO₂ are removed according to equation (2) and the residual NO₂ decomposes according to equation (3). However, the reaction of equation (3) is so slow that a part of the NO₂ will remain unreacted and flow downstream. Thus, while NO can be removed satisfactorily, NO₂ is difficult to remove. Consequently, the conventional denitration system has the disadvantage that its denitration performance is lowered as the proportion of NO₂ in NO_(x) becomes higher.

OBJECT AND SUMMARY OF THE INVENTION

In view of the above-described existing state of the art, it is an object of the present invention to provide a method for removing NO_(x) efficiently, even from exhaust gas containing a high proportion of NO₂.

According to the present invention, there is provided a method for the denitration of exhaust gas from a gas turbine or internal combustion engine which comprises (a) reducing a part of the NO₂ constituting NO_(x) contained in the exhaust gas to NO by means of a reduction catalyst bed for reducing NO₂ to NO, the reduction catalyst bed being installed in flow communication with the exhaust gas outlet of the gas turbine or internal combustion engine, and (b) injecting NH₃ into the exhaust gas within an exhaust heat recovery unit and then decomposing and removing NO_(x) contained in the exhaust gas and now composed chiefly of NO, by means a denitration catalyst bed installed within the exhaust heat recovery unit to effect the catalytic reduction of NO_(x) with NH₃.

Specifically, in order to treat exhaust gas containing a high proportion of NO₂, a reduction catalyst bed for reducing NO₂ to NO is installed on the upstream side of a dry denitrator using NH₃ as a reducing agent so that the proportion of NO₂ in the whole NO_(x) at the inlet of the dry denitrator may be lowered to facilitate the efficient removal of NO_(x). The proportion of NO₂ in the whole NO_(x) is preferably lowered to 70% or less and more preferably to 50% or less.

Since it has been found that the use of a reduction catalyst causes NO₂ present in exhaust gas to be partially reduced to NO, a reduction reaction represented by the following equation (4) can be effected by installing the above-described reduction catalyst bed on the upstream side of the dry denitrator in the flow of the exhaust gas.

    2NO.sub.2 →2NO+O.sub.2                              ( 4)

This lowers the proportion of NO₂ at the inlet of the dry denitrator. As a result, the reaction represented by the above equation (3) is suppressed to the fullest extent, making it possible to remove NO_(x) efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram illustrating one embodiment of the present invention;

FIG. 2 is a graph showing the temperature dependence of the degree of conversion of NO₂ to NO achieved by using an NO₂ reduction catalyst for reducing NO₂ to NO;

FIG. 3 is a graph showing the degree of denitration as a function of the proportion of NO₂ in NO_(x) ; and

FIG. 4 is a schematic flow diagram illustrating a conventional denitration system for exhaust gas from a gas turbine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to explain the effects of the present invention, the removal of NO_(x) in a combined plant provided with a means for reducing NO₂ to NO in accordance with one embodiment of the present invention is discussed below with reference to FIG. 1. An NO₂ reduction unit 11 for reducing NO₂ to NO is installed in flow communication with an exhaust gas duct 2 from a gas turbine or internal combustion engine 1. Connected to the downstream side thereof is an exhaust gas boiler 3. This exhaust gas boiler 3 comprises a plurality of heat exchangers 8, 9 and 10, an NH₃ injection nozzle 6 disposed between heat exchangers 8 and 9, and a denitrator 7 disposed between heat exchangers 9 and 10 and packed with a NO_(x) reduction catalyst. Furthermore, an exhaust gas duct 4 and a smokestack 5 are installed on the downstream side of the exhaust gas boiler 3.

Exhaust gas produced in the gas turbine 1, which contains large quantities of NO and NO₂, is introduced through the exhaust gas duct 2 into the NO₂ reduction unit 11 packed with an NO₂ reduction catalyst 12 for reducing NO₂ to NO, where most of the NO₂ present in the exhaust gas is reduced to NO. Thereafter, the exhaust gas is introduced into the exhaust gas boiler 3 disposed downstream, where it is cooled by the heat exchanger 8 and 9 to a low temperature suitable for denitration reactions. At the same time, NH₃ is injected from the NH₃ injection nozzle 6 into the exhaust gas and mixed therewith. Then, NO_(x) (i.e., NO and NO₂) is removed by the denitrator 7 packed with a denitration catalyst. Finally, the exhaust gas is conducted through the exhaust gas duct 4 and discharged through the smokestack into the air.

The temperature of exhaust gas produced in the gas turbine 1 is in the range of 500° to 600° C. and remains equally high in the NO₂ reduction unit 11. One example of the NO₂ reduction catalyst 12 for reducing NO₂ to NO, which is packed in the NO₂ reduction unit 11, is a catalyst prepared by wash-coating an inorganic oxide ceramic substrate (such as cordierite or mullite) with a porous carrier (such as alumina, silica or zeolite) and supporting thereon an active metal, such as platinum alone or platinum and at least one metal selected from the group consisting of nickel, cobalt, palladium, rhodium, vanadium, tungsten, iridium, iron and rubidium.

The degree of conversion of NO₂ to NO achieved by using the above-described NO₂ reduction catalyst is shown in FIG. 2. As can be seen from FIG. 2, the degree of conversion is about 60% at 500° C., about 70% at 550° C., and about 76% at 600° C. Higher degrees of conversion are achieved at higher temperatures. Accordingly, the NO₂ reduction unit 11 should be installed immediately following the gas turbine 1.

EXPERIMENT 1

A catalyst composed of about 80% TiO₂, about 10% WO₃ and about 2% V₂ O₅ with the balance being a forming aid was used in an atmosphere composed of 15% O₂, 6% CO₂, 6.7% H₂ O, 100 ppm NO_(x) and the balance N₂ and having a flow rate of 315N liters/hour and a temperature of 300° C. The proportion of NO₂ in NO_(x) was varied to evaluate its influence on denitration performance. As a result, it was found that, as shown in FIG. 3, the denitration performance began to lower when the proportion of NO₂ in NO_(x) exceeded 50% and the lowering tendency became more pronounced when it exceeded 70%.

EXPERIMENT 2

A catalyst was prepared by using a catalyst carrier comprising a Cordierite honeycomb substrate wash-coated with γ-alumina, and supporting platinum thereon in an amount of 2 g per liter of the catalyst carrier. Using this catalyst, the effect of converting NO₂ to NO was examined in an atmosphere composed of 50 ppm NO₂, 16% O₂, 2% CO₂, 5% H₂ O and the balance N₂ and having a flow rate of 600N liters/hour and a temperature of 500°-600° C. As a result, degrees of conversion of 50% or greater were achieved as shown in FIG. 2. Thus, it has been confirmed that conditions in which NO_(x) is efficiently removed can be achieved by installing a dry denitrator using NH₃ as a reducing agent on the downstream side of the NO₂ reduction unit.

In connection with Experiment 1, the relationship between the NO₂ /NO_(x) ratio and the denitration performance is shown in FIG. 3. Specifically, when the NO₂ /(NO+NO₂) ratio is in the range of 0 to 50%, the degree of denitration is 95% or greater. The degree of denitration is about 90% at a NO₂ /(NO+NO₂) ratio of 70%, and becomes lower at higher NO₂ /(NO+NO₂) ratios. Consequently, if the NO₂ reduction catalyst is used in such an amount as to give a NO₂ /(NO+NO₂) ratio of not greater than 70% and preferably not greater than 50%, the degree of denitration in the NO_(x) denitration unit disposed downstream of the NO₂ reduction unit in the exhaust gas flow will always be maintained at 90% or greater, thus making it possible to remove NO_(x) more efficiently. 

We claim:
 1. A method for the denitration of an NO_(x) -containing exhaust gas including NO₂ from a gas turbine or an internal combustion engine which comprises:(a) reducing part of the NO₂ present in the exhaust gas to NO by means of a reduction catalyst bed for reducing NO₂ to NO, wherein the reduction catalyst bed comprises platinum supported by a carrier comprising at least one porous material selected from the group of alumina, silica, and zeolite, and wherein the reduction catalyst bed is installed in flow communication with an exhaust gas outlet of the gas turbine or internal combustion engine in such a way that the conversion to NO takes place in the catalyst bed at a gas temperature between 500° and 600° C., and (b) injecting NH₃ into the exhaust gas from step (a) within an exhaust heat recovery unit and then decomposing and removing residual NO_(x) and formed NO contained in the exhaust gas by means of a denitration catalyst bed installed within the exhaust heat recovery unit to effect catalytic reduction of residual NO_(x) and formed NO with NH₃.
 2. The method of claim 1 wherein the reduction catalyst bed for reducing NO₂ to NO causes a proportion of NO₂ in the NO_(x) contained in the exhaust gas to be lowered to 70% or less.
 3. The method of claim 1 wherein the reduction catalyst bed for reducing NO₂ to NO causes a proportion of NO₂ in the NO_(x) contained in the exhaust gas to be lowered to 50% or less.
 4. The method of claim 1 wherein the reduction catalyst bed is directly connected with said exhaust gas outlet of the gas turbine or internal combustion engine.
 5. The method of claim 1 wherein the carrier for the reduction catalyst bed for reducing NO₂ to NO is prepared by wash-coating an inorganic oxide ceramic substrate with said at least one porous material.
 6. The method of claim 5 wherein the catalyst bed is composed of platinum and at least one metal selected from the group consisting of nickel, cobalt, palladium, rhodium, vanadium, tungsten, iridium, iron and rubidium. 